The manufacture of wet-laid acoustical panels includes a wet process having separate dilute water streams of fiber, fillers and binder which are then mixed together to create a slurry. The fibers are either organic or inorganic. Usually the fibers are inorganic for fire resistance. A typical binder is starch. Fillers may include newsprint (which also acts as a binder), clay, and perlite. A typical panel wet-forming process involves the successive steps of depositing a wet slurry onto a conveyor screen, draining water from the slurry through the screen including the use of suction for further water removal, compressing out additional water by means of a roll press and finally hot air drying the resulting wet panel as it is cast on the screen. Upon entering a drier, the wet panel typically has a 60 to 70% water content.
One of the most important aspects of a ceiling board is its sound absorption function. Artisans have employed many different techniques to increase sound absorption of acoustic panels, including apertures, fissuring, and striating. The relative noise reduction capability is expressed in terms of a Noise Reduction Coefficient (NRC). Historically, wet processed acoustical panels have not had a very high NRC as compared to dry processed ceiling boards, such as those made from fiberglass batts. However, there are many disadvantages associated with the use of fiberglass. Disadvantages include the cost of the fiberglass relative to natural fibers, complexity and costs associated with manufacturing fiberglass acoustical panels with organic binders, health and environmental concerns associated with the use of organic solvents and organic binders in the manufacture of fiberglass acoustic panels, and the lack of strength associated with acoustic panels having inner cores comprised of fiberglass batts.
In the manufacture of wet-processed acoustical panels, it is desirable for the sound absorbent composite materials to achieve an acceptable level of sound absorbency. This is usually done by reducing the density of the panel or increasing the panel thickness. Competing with the requirement of high acoustical absorbency is the need for a relatively stiff material to provide sufficient structural integrity and sufficient surface hardness to resist punctures and dents which may occur during the manufacture, transport, installation or use of the product. Additionally, a minimum thickness is also desirable to lower the material cost associated with the manufacture of acoustical panels from the acoustically absorbent material.
Unfortunately, wet-processed materials that exhibit sufficient stiffness and surface hardness are usually quite dense, have small and closed pores, and therefore do not display acceptable sound absorption characteristics. Furthermore, wet-processed materials with highly acoustical absorbent properties are much less dense due to increased porosity, and therefore do not exhibit sufficient stiffness and surface hardness properties required for acoustic panel applications. Additionally, since traditional wet-processing techniques require a vacuum drawn through a cross-section of the wet-laid material to remove water, a significant porosity size gradient arises through a cross-section of the panel, which further degrades the acoustic attenuation properties and strength of the finished panel.
With the foregoing problems in mind, it is an object of the present invention to create a novel acoustical panel having a high acoustic absorbency by altering the shape of a surface of the panel to achieve both good sound reduction and to retain excellent strength properties.